This application is submitted to the NIMH DATR Mood/Sleep Research Program A2-AID. Depressive disorders occur at a high rate in patients with inflammatory disorders, with a point prevalence of 15-29%, which is two to three times greater than that observed in the general population. Substantial evidence has shown that inflammation and increases in proinflammatory cytokine activity play a critical role in the onset and perpetuation of depression and depressive symptoms (e.g. insomnia, fatigue) in those who are co-morbid for inflammatory disorders (Miller et al., 2009). Consistent with this, experimental work has shown that an inflammatory challenge can increase depressed mood in an otherwise healthy sample (Reichenberg et al., 2001). Based on these findings, there has been a growing interest in whether inflammatory processes can contribute to depression in a causal manner and how these effects might occur. Given the observation that inflammatory processes trigger social withdrawal (Dantzer, 2001; Hart, 1988), coupled with evidence that feelings of 'social disconnection' play a critical role in the onset and perpetuation of (non-inflammatory forms of) depression (Heinrich & Gullone, 2006), it is surprising that the social psychological consequences of inflammation and their contribution to depression have not been more fully explored. Here, we suggest that inflammation may increase feelings of social disconnection and that these social psychological changes may be an important contributor to inflammation-associated depression. Indeed, preliminary data demonstrated that an experimentally-induced inflammatory challenge (endotoxin) led to increases in self-reported feelings of social disconnection (e.g., I feel disconnected from others) in addition to increases in depressed mood (Eisenberger et al., 2009b). Aside from these findings, however, there are no studies that have explored the effect of inflammatory processes on social experience in humans. The over- arching objective of this proposal is to explore the experiential and neural correlates of inflammatory- induced changes in social experience (e.g., feelings of social disconnection), which may provide a critical missing link in understanding the relationship between inflammation and depression. Participants (n=100) will be randomly assigned to receive either endotoxin or placebo and will then be monitored for the next six hours. Blood draws to assess cytokine levels as well as self-reported feelings of social disconnection and depressed mood will be collected hourly. In addition, at the time of peak cytokine response, participants will complete a neuroimaging session to examine the effect of inflammatory challenge on neural sensitivity to social rejection and social acceptance. It is hypothesized that endotoxin will increase feelings of social disconnection over time, and that the underlying neural sensitivities that give rise to these feelings (e.g., increased neural sensitivity to social rejection; decreased neural sensitivity to social acceptance) will contribute to inflammatory-induced depressed mood.